Holder faced off against House committee chairman Darrell Issa, R-Calif., who asked why the investigation into the Justice Department's flawed gun-smuggling probe was taking so long.
Issa demanded the Justice Department turn over documents about how it handled congressional inquiries after problems with Fast and Furious came to light 13 months ago.
"This committee has lost its patience to wait longer," Issa said.
In Fast and Furious, agents lost track of nearly 1,400 of the more than 2,000 guns purchased by suspected straw buyers. Some 700 guns connected to suspects in the operation have been recovered in Mexico and the U.S., some at crime scenes, including the one near Nogales, Ariz., where border agent Brian Terry was murdered in December 2010.
The attorney general said the department was inclined to follow longstanding tradition of withholding internal documents about congressional inquiries in order to preserve the ability to get candid advice from top officials.
"I think you're hiding behind something here," Rep. Dan Burton, R-Ind., told Holder. "You ought to give us the documents. ... It appears we're being stonewalled."
Issa has threatened contempt of Congress ruling against Holder for failing to turn over the subpoenaed documents.
Holder says an internal investigation is being completed, and he expects firings and charges against officials within six months.
"This has become political, that's fine," Holder said later at the hearing, but there is no attempt "at a cover-up." The Justice Department, Holder insisted, "will continue to share huge amounts of information."
The Associated Press contributed to this report.